In theory, it could be VBR if it's packed inside another container format such as MP4, AVI, MKA, etc. However, such a scheme would be non-standard and would require modifications to existing decoders to play properly

In theory, it could be VBR if it's packed inside another container format such as MP4, AVI, MKA, etc. However, such a scheme would be non-standard and would require modifications to existing decoders to play properly

In theory, it could be VBR if it's packed inside another container format such as MP4, AVI, MKA, etc. However, such a scheme would be non-standard and would require modifications to existing decoders to play properly

Isn't it mp3 stream in mp4 container a standard? I thought so...

It is. But for such a scheme to work, decoders would have to rely on the size stored for each "sample" (packet) in the MP4 mdat, rather than trying to parse it from the MP3 header. I'm not aware of any MP3 decoder APIs that are capable of this, although it would be trivial to add.

Newer codecs (AAC, Vorbis) are more robust in every way, they've got a better quality-to-filesize ratio. The only reason people still use MP3 is because it's compatible with everything and will play everywhere.

When you go to free-format, you've broken that compatibility, and there's no reason anymore to stay locked in to MP3. Might as well go to better codecs if you're going to break that compatibility. So there's very little incentive for anyone to improve upon freeformat encoding.